Regenerative amplifiers which have heretofore been proposed do not incorporate a spectral filter element within the cavity of the regenerative amplifier. Typically, systems lacking this feature are run at the peak of the gain profile, where the bandwidth of the seed laser dominates and thus swamps out self-oscillation within the resonant cavity of the amplifier itself. When the seed pulse is tuned to a wavelength that is substantially off the peak of the gain profile of the amplifying medium, there is no longer any mechanism to prevent a significant fraction of the energy available in the gain medium from being removed through the mechanism of self-oscillation in the amplifier resonant cavity. This is undesirable because, while the seed pulse may still be amplified to some degree, the output energy is degraded because of loss of energy through this parasitic process of self-oscillation. Additionally, since the off-peak amplified seed pulse is now incorporated within this self-oscillation emission, the temporal nature of the process being studied using these pulses becomes far more difficult to unravel. By incorporating a spectral filter element within the resonant cavity of the regenerative amplifier it is possible to suppress self-oscillation, and thus allow for greater amplification of only the seed pulse. This produces a cleaner, more useful output pulse. It also produces a pulse with the desirable feature that it has a time-bandwidth product (TBWP) that is nearly transform-limited.
When a regenerative amplifier is run in a mode that is optimized to amplify pulses of extremely short pulse duration, typically much less than 100 fs full width half maximum FWHM gain narrowing can limit the system's ability to maintain these extremely short pulse widths. By incorporating one or more suitably designed and oriented spectral filters within the regenerative amplifier cavity, it is possible to shape the system's effective gain profile to preserve the pulse width of the seed pulse throughout the amplification process. Another advantage to the use of a suitably designed and oriented spectral filter or filters in a regenerative amplifier cavity is it can act as a passive protection mechanism that prevents catastrophic damage due to self-focusing.
Initial attempts to run regenerative amplifier systems designed to amplify seed pulses of femtosecond (fs) or near femtosecond duration (that is, pulses of full width half maximum [FWHM] duration ranging from 1 to several hundred fs) so as to produce longer pulses of several hundred femtosecond and even picosecond (ps) duration (that is pulses of duration FWHM ranging from 0.1 ps to 1000 ps), which are also transform-limited, have used a bandwidth-limiting element within the stretcher or compressor portion of the system itself, and not within the resonant cavity of the regenerative amplifier. Heretofore it had been believed that limiting the bandwidth in the resonant cavity of the regenerative amplifier should be avoided because doing so would create pulses of such short duration that they would be amplified to pulse energies that exceed the self-focusing threshold in the amplifier. Regenerative amplifiers operate on the principle of chirped pulse amplification wherein the injected pulse has a pulse width whose duration is stretched out in time before being injected into the amplifier resonant cavity. In this manner the self-focusing threshold, which, when exceeded can result in catastrophic damage to components therein, is avoided upon amplification. However, the chirp created on the injected pulse by the stretcher is typically arranged so that the variation in frequency is mostly linear over the duration of the pulse. Consequently, limiting the bandwidth in the amplifier resonant cavity would also result in a reduction of the pulse width of the injected pulse within the amplifier itself, and thereby place the amplifier components at risk of damage due to self-focusing.
The elements used to limit the seed pulse bandwidth within either the stretcher or compressor, or both, usually take the form of a slit which is used to aperture the beam, and consequently the spectrum, at a position where the spectrum of the seed pulse is horizontally dispersed. The strategic placement of this slit within the stretcher or compressor, and the appropriate adjustment of its width, limits the bandwidth of the pulse seeded into the regenerative amplifier, and consequently the bandwidth of the output pulse. This creates a pulse of longer temporal duration. However, a slit placed in the stretcher does not guarantee that the output pulse from the system will be substantially transform-limited, because when the system is run off the peak of the gain curve, frequency pulling in the amplifier itself will shift the center wavelength of operation back toward the gain peak. Additionally, this approach has the disadvantage that the spectral profile of the seed pulse is now cut off in a manner that creates satellite pulses. Both of these are undesirable features.
A variant on the "slit within the stretcher" technique described above uses a "soft-edge" mask whose sides possess a slowly varying transmissivity or reflectivity as a function of position (depending on whether the mask is used in transmission or reflectivity to limit the spectral bandwidth of the pulse seeded into the regenerative amplifier). While this approach can ameliorate the problems associated with the creation of satellite pulses in the output of the regenerative amplifier, it still cannot compensate for frequency pulling effects that may result in spectrally broadened pulses when the system is run off the peak of the gain curve.
Using either a slit aperture or "soft-edge" aperture in the compressor avoids many of these problems, but this technique suffers from poor throughput, because the energy that is contained in the blocked frequencies is lost.
This invention is particularly suited for use in providing amplified pulses that are free of unwanted emissions at wavelengths other than those that are associated with the seed pulse. For example, this invention results in a significant reduction of unwanted self-oscillation emission at the peak of the gain from the resonant cavity of the regenerative amplifier when the seed pulse being amplified has a central wavelength that is substantially different from that corresponding to the peak of the gain of the amplifying medium. An additional advantage is that the presence of a spectral filter element whose parameters are judiciously chosen within the regenerative amplifier resonant cavity can produce a bandwidth-limiting effect that forces the regenerative amplifier to produce output pulses that are substantially longer in duration than has been possible before and have the desirable characteristic of being nearly transform-limited. Such pulses are useful in research applications where not only must the central wavelength be tunable to a specific transition wavelength of interest, but the spectral bandwidth must be limited in order to avoid pumping adjacent excited states. Moreover, when an amplifier according to this invention is seeded with pulses of broad spectral bandwidth as, for example, from an oscillator producing pulses of tens to hundreds of femtosecond duration, said judiciously chosen spectral filter element can function as a fine control over the central wavelength of oscillation of a long pulse amplifier. This allows a regenerative amplifier system's output to be tuned over a large fraction of the spectral emission band of the seed oscillator by adjustment of only the spectral filter element.
An additional advantage of incorporating a spectral filter element within the resonant cavity of a regenerative amplifier is that through the judicious choice of the parameters of the spectral filter element so as to limit the bandwidth of the pulse being amplified, it is possible to obtain transform-limited pulses of picosecond and near picosecond duration that are free of satellite pulses from a system capable of amplifying pulses of femtosecond duration. This feature significantly increases the number of scientific problems this invention can be used to address, and so the utility of the invention itself.
Yet another advantage of incorporating a spectral filter element within the resonant cavity of a regenerative amplifier is that it can serve as a passive protection mechanism that prevents catastrophic damage due to self focusing. The onset of self-focusing is accompanied by self-phase modulation and other nonlinear effects which broaden the spectral width of the pulse. The spectral limiting effects of a spectral filter in the regenerative amplifier cavity will block the further amplification of this excess bandwidth, thus effectively clamping the bandwidth at a level that prevents damage to the components of the system.
The aforementioned advantages of using a spectral filter in a regenerative amplifier cavity assumes that the filter is used in a configuration in which its transmission maxima affects the intended result. There are, however, conditions in which one may choose to run the spectral filter in a configuration in which its transmission minima is employed to produce the desired effect. For example, when amplifying extremely short seed pulses, it may be desirable to make use of the filter's transmission minima to alter the system's effective gain profile so as to reduce or minimize the effects of system gain narrowing on the output pulsewidth. Thus, if the regenerative amplifier were to possess an effective gain profile as a function of wavelength that was approximately gaussian in shape, the use of a spectral filter with a low finesse, approximately sinusoidal transmission function whose transmission minima was tuned to coincide with the peak of the gaussian gain profile would somewhat flatten the top of the gain curve and allow for the redistribution of energy into the spectral wings of the pulse being amplified. The effect would be to reduce gain narrowing in the amplification process and result in shorter pulse duration output pulses. Combinations of spectral filters operated in both transmission maxima, transmission minima, and detuned modes will result in pulse shapes that are desirable to affect many different output pulse shapes.
Accordingly, it is an object of the present invention to provide improved regenerative amplifier systems which are free of emission due to self-oscillation from the resonant cavity of the amplifier itself.
It is a further the object of the present invention to provide bandwidth-limited operation of regenerative amplifier systems that provide pulses with nearly transform-limited, time-bandwidth product performance within the entire tuning range of the gain medium.
It is a still further object of the present invention to provide improved regenerative amplifier systems with bandwidth-limited, nearly transform-limited, time-bandwidth product pulses of sub-picosecond and/or picosecond duration when seeded with pulses obtained from oscillators that are of femtosecond duration.
It is a still further object of the present invention to provide improved sensitivity and resolution in tuning the central wavelength of the output pulse from regenerative amplifier systems so that the central wavelength of the output pulse can be controlled with high precision.
It is a still further object of the present invention to provide improved regenerative amplifier systems with bandwidth-limited, nearly transform-limited, time-bandwidth product pulses of sub-picosecond and/or picosecond duration that are free of satellite pulses when seeded with pulses obtained from oscillators that are of femtosecond duration.
It is a still further object of the present invention to provide improved protection against catastrophic failure due to self-focusing in regenerative amplifier systems.
It is a still further object of the present invention to provide improved amplification of extremely short seed pulses by altering the effective gain profile as a function of wavelength of a regenerative amplifier.